The iso-paraffin/olefin alkylation process is widely used to manufacture a high octane quality blend component for aviation and motor gasoline which is also valued for its relatively low vapor pressure, low sensitivity and, because of its freedom from aromatic components, its environmental acceptability. The process typically reacts a C2 to C5 olefin with isobutane in the presence of an acidic catalyst to produce the alkylate product.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal but in spite of efforts to develop an inherently safe alkylation process, both processes have achieved widespread utilization with the HF process being noted for producing a higher quality product with more favorable unit economics. Although hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive, extensive experience in its use in the refinery have shown that it can be handled safely, provided the hazards are recognized and precautions taken. The HF alkylation process is described in general terms in Modern Petroleum Technology, Hobson et al (Ed), Applied Science Publishers Ltd. 1973, ISBN 085334 487 6. A survey of HF alkylation may be found in Handbook of Petroleum Refining Processes, Meyers, R. A. (Ed.), McGraw-Hill Professional Publishing, 2nd edition (Aug. 1, 1996), ISBN: 0070417962.
In order to improve the operation of the HF alkylation process as well as the economics of the process it is desirable to regenerate the HF alkylation acid by removing the polymeric by-product which forms during the alkylation reactions; this polymer, comprising polymers of differing degrees of conjugation, is frequently referred to as “acid soluble oil” (ASO) since it is miscible with the HF acid phase. Removal of the ASO is necessary to preserve the concentration of the acid at the high level desirable for good alkylation performance while removal of water is required in order to reduce corrosion within the unit as well as to maintain product octane quality. Normally, the acid concentration is maintained at 88 to 94 weight percent by the continuous or periodic addition of fresh acid and withdrawal of spent acid with the water content kept in the range of 0.5 to 1.0 percent.
One previously used method for removing the polymer from the acid inventory is by internal regeneration. A small amount of acid (with polymer and trace water) is injected into the isostripper feed line with the large amount of alkylate and butanes feeding the tower. The acid flashes overhead and the polymer leaves with the alkylate via the bottom of the tower. There is, however, a limit on the amount of the acid in the hydrocarbon feed to the isostripper: major corrosion problems in the isostripper overhead may occur with excess acid in the isostripper feed. Excess acid can also result in a separate acid phase in the downstream isoparaffin recycle circuit where it mixes with olefin feed, resulting in accelerated corrosion in the tower.
When removal of water is the objective, as it is on a fairly frequent schedule, external regeneration is necessary. In the external regeneration method, a small amount of acid (with polymer and trace water) is injected into a separate distillation column, which is operated in batch mode with isoparaffin used as a heating and stripping medium. The tower separates dry acid overhead and a mixture of HF acid, water and polymer leaves as a bottoms fraction. The mixture is neutralized, and the aqueous phase is separated from polymer through a series of separator drums. This method results in loss of HF acid and costly use of neutralization chemicals. It is also manually intensive.
An external regeneration method of this type is described in U.S. Pat. No. 5,547,909 (Carlson). In this method, the acid phase from the settler is removed and a portion is routed to a separator column which enables the polymer (ASO) to be separated from the HF of the acid phase. Cooled isoparaffin is used as column reflux and heat is supplied by means of isoparaffin introduced as stripping medium at the foot of the column. Acid, free of ASO and water is removed as overhead and is recirculated to the settler. A portion of the bottoms fraction is recycled to a higher level in the tower, possibly to make the separation more effective. While this technique may be capable of improving on the conventional external separation by attempting to get closer to an azeotropic mixture of water, acid and polymer in the bottom of the tower, (acid content of the bottoms stream about 40-50%), it still fails to achieve a satisfactory level of acid recovery and significant losses of acid can be expected. Another external HF acid regeneration scheme is proposed in U.S. Pat. No. 6,228,650 (Moore et al) using an improved control scheme to monitor and control process operation but again, since the same basic regeneration scheme is used in which the heat is supplied solely by the iso-paraffin used for stripping, the acid regeneration remains at a level which is less than optimal. A method of stripping the HF from the alkylation acid using hydrogen is proposed in U.S. Pat. No. 5,461,183 (Del Rossi et al) but this method, requiring external hydrogen is not well suited to incorporation within the non-hydrogenative HF alkylation unit.
There are therefore deficiencies in both methods of regeneration: internal regeneration in the isostripper removes polymer but is ineffective for removal of water to the low levels required in the process while external regeneration, by contrast, is required for the removal of water but results in an uneconomic loss of HF acid if the water content is to be maintained at the appropriate level. Given that the process economics favor internal regeneration, it would be desirable to improve the operation of that practice even if it were not completely effective at removal of water.
We have now devised a method of HF acid regeneration which enables the polymer to be separated from almost all of the acid, together with water, leaving the polymer ASO with only trace acid in the alkylate product as a bottoms fraction. The method we have devised operates by using the heat from a stream of hot alkylate to strip the acid, together with trace levels of water, from a slipstream of alkylation acid with its polymer (and trace water) in an acid treating tower (fractionation tower) in which the acid and water are vaporized as overhead, leaving the polymer behind as a bottoms fraction which leaves the tower with recycled alkylate. This polymer-laden alkylate is injected into the isostripper feed to remove any trace residual acid as isostripper overhead. The HF acid, water and light hydrocarbon vapors from the treating tower are condensed into an overhead receiver drum, where light hydrocarbon is separated from the acid phase. The remaining acid and trace water is virtually polymer free and can be returned to the circulating acid system or fed to an external regeneration unit for removal of water, when required. The acid treating tower allows for elimination of internal regeneration as well as a reduction in external regeneration. In addition, since the acid treating tower configuration sends only acid (and trace water) to the external regeneration unit, the costs associated with extensive handling of polymer in external regeneration are eliminated. Because water and acid are largely removed in the acid treating tower, the problems of isostripper overhead corrosion from excess acid are largely eliminated.